1. Field of the Invention
This invention relates to a method and apparatus for efficiently and robustly measuring gas concentrations/partial pressure of respiratory and anesthetic gases.
2. Description of the Related Art
It is well known by those of ordinary skill in the art that gas analyzers of the nondispersive infrared (NDIR) type operate on the principle that the concentration of specific gases can be determined by (a) directing infrared radiation (IR) through a sample of a gaseous mixture, (b) separately filtering this infrared radiation to minimize the energy outside the band absorbed by each specific gas (c) measuring the filtered radiation impinging upon one or more detecting devices and (d) relating a measure of the infrared absorption of each gas to its concentration. Gases that may be measured exhibit increased absorption (and reduced transmittance) at specific wavelengths in the infrared spectrum such that, the greater the gas concentration, the proportionally greater absorption and lower transmittance. An extension of this NDIR technique uses a continuous, linear bandpass filter, followed by a linear array of detectors.
Gas analyzers are widely used in medical applications and may be characterized as being located either in the main path of the patient's respiratory gases (mainstream analyzers) or in an ancillary path usually paralleling the main path (sidestream analyzers). A mainstream analyzer is situated such that the subject's inspired and expired respiratory gases pass through an airway adapter onto which the analyzer is placed. Mainstream designs require the optical and electronic components to be interfaced to a patient's airway or to a respiratory circuit in communication with a patient in a location in relatively close proximity to the patient. As a result, to be accepted in clinical use, the mainstream gas analyzer must be designed as a compact, lightweight yet robust structure unaffected by typical mechanical abuse and temperature variations associated with prolonged use in health care facilities.
While conventional mainstream gas analyzers work well for a small number of specific, non-overlapping spectrum wavelengths, it is difficult to change wavelengths of interest. The system becomes increasingly inefficient if there are more than 2 or 3 wavelengths of interest, and it is very difficult and expensive to provide resolutions significantly better than 0.1 micron, FWHM (full-width at half maximum) in the IR region.
It is known to use grating spectrometers for gas analysis. There are two general configurations of grating spectrometers: the spectrograph, which originally spreads the spectrum out over a strip of photographic film or a linear array detector, and the spectrometer, which uses a single detector that is set at an appropriate location or angle to register a particular spectral element.
For IR gas measurements, an IR source provides broadband energy that is collimated and passed through a gas sample cell. The collimated broadband energy, now attenuated at certain wavelengths, is directed to a diffraction grating where it is diffracted from the grating, spread out into a continuous spectrum, and focused with a mirror onto a small detector. The diffraction grating is rotated about an axis parallel to the grating lines, and substantially coaxial with the face of the diffraction grating. As the diffraction grating is rotated, the spectrum is scanned past the single detector. Since the diffraction grating rotation is synchronized with the detector readout electronics, specific, but arbitrary, spectrum features can be isolated and registered.
It is axiomatic that a microspectrometer should be small and lightweight. The present invention contemplates, for example, that the microspectrometer is made small and lightweight enough to be used directly on a patient airway, i.e., mounted in a mainstream fashion on a patient circuit. While the optics can, in general, be made small enough to suit the purpose, it is difficult to make the mechanism that drives the diffraction grating, that is, the spectrum scanner, sufficiently small to suit this purpose. Currently available electro-mechanical scanner drives that are much too large, mostly too heavy, require too much power, and cost too much to be used in this manner.
For example, many conventional spectrometers rotate the diffraction grating using a motor of some sort, oscillating linkages to drive the diffraction grating from the motor, and a bearing assembly. While such an arrangement can deliver good results, such a structure is relatively large, heavy and expensive. Other conventional spectrometers use an oscillating motor, sometimes called a galvanometer drive, in place of the motor and linkage. Such arrangements are less expensive, but still large, heavy and relatively expensive.
U.S. Pat. Nos. 6,249,346 (2001) to Chen, et al., 6,039,697 (2000) to Wilke, et al., and 5,931,161 (1999) to Keilbach, et al. all disclose relatively smaller sized spectrometers, but of designs that are of undue bulk and, in some instances, complexity.